The drive for high performance involves high speed operation of microelectronic components having high drive currents in addition to low leakage, i.e., low off-state current, to reduce power consumption. The structural and doping parameters tending to provide an increase in drive current adversely impact leakage current.
Gate last metal gate, also called replacement gate, techniques have been developed to address problems attendant upon substituting metal gate electrodes for polysilicon gate electrodes. For example, an amorphous silicon (a-Si) or polysilicon gate is used during initial processing until high temperature annealing to activate source/drain implants has been implemented. Subsequently, the a-Si or polysilicon is removed and replaced with a metal gate.
A number of issues are present with replacement metal gates.
For example, during work function metal formation, the N-type field effect transistor (NFET) is filled by an organic planarization layer (OPL) and etched away by a patterning process. During the OPL etch process, the long channel of the transistor is opened earlier than the short channel due to a reactive ion etch (ME) loading effect. In such cases, the ME causes damage on the long channel, thus leading to transistor performance degradation.
Another problem with replacement metal gate formation results from the combined work function metal in NFET (NFET work function first) or PFET (PFET work function first) in the short channel. Because there has to be a smaller space, or no space, to fill by gate W metal in one polarity, larger gate line resistance results.
Furthermore, conventional work function chamfering processes, such as performing an OPL coating followed by lithography to cover long channel, as well as a partial OPL recess on the short channel followed by an OPL strip, involves an additional lithography process to protect long channel work function metal. The additional lithography process increases processing steps and therefore cost.